Interpretive Summary: Lycopene accumulation genes are well defined in the model fruit, tomato. The regulatory mechanisms leading to accumulation of other carotenoids and flesh colors, however, are poorly understood. The multiple flesh colors available in watermelon fruit (red, orange, salmon yellow, and canary yellow) makes it ideal for investigating the regulation of these other carotenoids. Carotenoid accumulation was measured in seven watermelon varieties, representing the four flesh colors, throughout fruit maturation. It was found that in general, products downstream of the major carotenoid (for a given flesh color at maturity) appears to be down-regulated, causing an accumulation of the major carotenoid and its precursors. Additionally, seedless varieties were generally found to have higher levels of carotenoids than seeded varieties.

Technical Abstract:
The carotenoid biosynthetic pathway regulatory mechanisms leading to lycopene accumulation are well defined in the model fruit, tomato (Lycopersicon esculentum L.). The regulatory mechanisms leading to accumulation of other carotenoids and flesh colors, however, are poorly understood. The variety of flesh colors available in watermelon (Citrullus Lanatus (Thunb.) Matsum. & Nanai) fruit (red, orange, salmon yellow, and canary yellow) makes it ideal for investigating the regulation of the full pathway. Carotenoid accumulation was measured in seven watermelon varieties, representing the four flesh colors and two ploidy levels, throughout fruit maturation. It was found that the putative regulatory mechanisms controlling lycopene accumulation in red-fleshed fruit may act as a template for understanding regulation of the pathway leading to other flesh colors. We found that in general, products downstream of the major carotenoid (for a given flesh color at maturity) accumulate until around day 20, or the "breaker stage," at which time genes immediately following this carotenoid appear to be down-regulated, causing an accumulation of the major carotenoid and its precursors. But no obvious signs of feedback inhibition were detected. Additionally, triploid varieties were generally found to have higher levels of carotenoids than diploids (an average of 1.5 - 4.0 times higher in red and orange flesh, respectively). This broadened understanding of the regulation of the full biosynthetic pathway is critical in planning traditional breeding and biotechnological manipulation techniques of carotenoid-containing species.